Thermal ink jet print heads are commonly used in a wide variety of printers. They operate by propelling a drop of ink from a nozzle in the print head in response to an electrical signal impulse. This is generally known as "drop on demand" printing. Typically, the print head will receive a signal in the form of an electrical current, which may be directed to a resistance heater element.
As the current passes through the resistance heater in a thermal ink jet print head, a small amount of ink on the surface of the heater element is heated. As the ink heats, a component of the ink, usually water, becomes superheated to the point that it vaporizes, creating a vapor bubble. The expansion of the vapor bubble produces a pressure pulse which imparts momentum to a portion of the ink, thereby propelling the ink through an ink ejection nozzle so that it impacts on the paper. By providing a plurality of ink ejection nozzles and heater elements associated therewith, and by timing the electrical signals to one or more heater elements, patterns of ink forming images, such as letters, can be produced on a substrate.
There are a number of factors which effect the quality of the images produced by a thermal ink jet print head. Among the factors are the characteristics of the resistance heaters, the properties of the ink and the geometry of the print head and ejection orifices. Printer manufacturers are constantly searching for techniques which may be used to improve print quality.
Print quality is related to how precisely the ejected ink droplet from the print head is placed on the substrate. Because the paper and the print head are typically moving with respect to each other as the ink is being ejected from the print head, the velocity with which the ink droplet is expelled from the print head orifice effects the placement of the droplet on the paper. It is traditionally believed that print quality is maximized by maximizing droplet velocity. Therefore, print head manufacturers have typically designed their print heads for maximum ink droplet velocity. One method of doing this is to control the heater power density at a point which achieves the maximum bubble wall velocity, which in turn imparts momentum to an ink droplet.
The power density of a resistance heater is the ratio between the amount of power sent to the heater, and the surface area of the heater. A graphical relationship between droplet velocity and power density shows that droplet velocity is maximized at a power density of between about 1.1 gigawatts per meter squared and about 1.7 gigawatts per meter squared of heater surface area. At power densities either greater than or less than this range, droplet velocity decreases. Thus, manufacturers typically design their print heads to operate within this range.
The power density of the heater also tends to have an inverse relationship with nucleation time, or in other words the time required to vaporize a portion of the ink. At relatively low power densities nucleation time is increased, and at relatively high power densities nucleation time is reduced.
A longer nucleation time is traditionally preferred, as more time is thereby provided to transfer energy from the heater to the liquid phase of the ink before the vapor phase separates the liquid ink from the surface of the heater element. By imparting more energy to the liquid ink, it is typically thought that bubble growth is better sustained, and produces a more consistent expulsion of the liquid ink, and hence better print quality. Therefore, print head manufacturers tend to design print heads that work at as low a power density as possible, yet at a power density just great enough to achieve the maximum drop velocity, as described above. An exception is the ExecJet IIc printer, which has a power density of 1.89 gigawatts per square meter (GW/m.sup.2). Although the power density of that printer is attributable to the inventor of this application, that printer is prior art as it has been sold for more than a year.
With regard to the foregoing, it is an object of the present invention to improve the print quality of a thermal ink jet printer.
Another object of the present invention is to reduce the drop placement variation of ink ejected from a thermal ink jet printer.
Still another object of the present invention is to provide an improved method for operating a thermal ink jet printer so that ink droplet placement variation is minimized.